1. Field of the Invention
The present invention relates to an eletrode which can be implanted into a living body for creating an electrical stimulation system. Such a system is used as a method for functional training or restoration of paralyzed muscles, and for the measurement of electrical signals of the living body, etc.
2. Description of the Prior Art
Because of disturbances in the neuromuscular system resulting from cerebrovascular disorders, spinal cord injury, or other injuries, investigations are presently being made to recover the lost function of body strength. Rehabilitation medicine, one side, and on the electrical stimulation (hereinafter referred to as "FES") on the other side are being investigated with the aim toward restoring or helping the disturbance of motor function which is not at all restorable through the mere employment of utensil or operations dependent on electrical stimulation from the outside.
Similarly, in the case of other than the above, sometimes in order to supply the electricity direct to the living body, and sometimes conversely in order to detect electrical signals from nerves and muscles of the living body, there is conducted the implantation of electrodes direct into the living body.
For example, the above-mentioned FES system, has already been proposed in the official gazettes; the Japanese Patent Application Disclosure No. 217174 of 1986 or the Japanese Patent Application Disclosure No. 108054 of 1985. Especially in the latter official gazette, the electode used for this purpose is made up of twisting together carbon fibers or stainless steel fine wires (for example, about 0.2 mm in diameter) coated on their surfaces with a resin.
On the other hand, the inventors, obtained electrodes offered for clinical purposes in accordance with the FES system, made by the A-M system corporation (U.S.A.), and constructed by intertwisting seven solution treated stainless steel wires.
This electrode is a thread-like filament material of about 0.6 mm in diameter, which is formed by coating the surface of the conductor 12 made by twisting together seven pieces of the above-mentioned stainless steel fine wires 11 with a resin 14 with the exception of its one end part 13, as illustrated in FIG. 5. The whole body of the formed electrode is formed with a coil shape.
This electrode 10 is inserted percutaneously into the living body A so that its one end part 13 is located near the nerve 18, and the other end is connected through the plug 15, the lead wire 16, and other elements to the electrical stimulation device 17. The device is so constructed so that an electric current is generated from the end part 13 of the electrode 10 to stimulate the above-mentioned nerve 18. Thus the structure of the FES system is completed. Reference number 19 in the figure is the guiding needle for inserting the electrode 10 into the living body A.
The present inventors examined many electrodes of such type with the result that when the conductor was covered with resin and implanted in the affected part of a living body, the following disadvantageous results attributable to the properties of material of the above-mentioned conductor itself were noted.
That is, in the electrode as disclosed in the above-mentioned official gazette: the Japanese Patent Application Disclosure No. 108054 of 1985, each of stainless steel fine wires contained therein was considerably large, i.e., about 0.2 mm in diameter and has a relatively large rigidity. This gives rise to a problem that the above-mentioned non-covered end portions tend to injure the living body when contacting directly with the nerve or muscle due to expanding/contracting or shifting of the electrodes in the body.
Such an electrode in facet heightens the physical burden of the patient himself due to the insufficiency is the flexibility of the electrode when it is used in such the regions, as the internal ear or the spinal cord which is very sensitive and susceptible to tissue lesion.
In this case, it has been suggested to use some soft wire with the object of making the above-mentioned fine wire soft. However, the use of soft wire makes it difficult to effectively treat the body because the wire readily changes in its form and further, the end part of the electrode tends to shift and deform as mentioned above. On the other hand, the above-mentioned fine wire tends to bread due to the repeated bending fatigue, thereby lowering the property of electrical conductivity.
On the other side, there is also disclosed in the above-mentioned official gazette the use of carbon fiber, however, the carbon fiber is generally inferior in flexibility. Therefore, when taking the problem of breakage into consideration, it is difficult to say that such a fiber may be used with reliability.